SCH 4C - Limiting & Excess Reactant

 Imagine the following scenario:

You work for a company that assembles bicycles.  Your job is to attach two wheels to a single frame.  When the supply truck came in to drop off inventory, your boss took delivery of 250 frames and 400 tires.  You assemble bicycles all week long and on Friday, the boss comes to inspect your work.  He is upset because you have not assembled 250 bicycles.  Can you explain to him why you haven’t used all the frames from the last delivery?

Of course, you would say that you ran out of tires.  With two tires per bicycle and 400 tires in inventory, the best you could do was to assemble 200 complete bicycles before running out of tires.  The tires limited how many complete bicycles could be built.  The extra, or excess, frames could not be used.

Chemistry works the same way. 

Reactants combine in specific ratios.  Sometimes, we run out of one reactant and the reaction stops working.  The reactant that runs out is called the limiting reactant.  It limits how much of the products can be made (like the tires in the above scenario). 

The reactant that doesn’t run out is called the excess reactant because it doesn’t get used up in the reaction (like the frames in the above scenario).

Let’s see this in action.  Check out the video and follow along:  A

ex. In a reaction, 15.8 g of aluminum is reacted with 55.6 g of bromine. 

(a)   Determine the identity of the limiting reactant.

(b)   Determine the mass of aluminum bromide produced.

As with all the other calculations we have done, format is vital, so be sure you use it.  In the photo above you can see how I grade a question like this, based on the check marks.

I need to see:

• data (knowns/unknowns), equation, substitution, answer, in neat columns under each substance
• include well-laid out mole ratios where appropriate
• include units with every number
• round to the correct sig digs at, & only at, the end of a section of the question 
• box your final answer

Correct answers are nice, but technique and format is everything!  Never leave me to guess how you worked through a question.  The more you give me, the more I can give you (in terms of part marks) if you mess up.

Homework: 

Practice, p. 153 # 3, 4

Section Questions, p. 153 # 1, 2, 6

Go over what you have learned about stoichiometry.  Be sure you understand how to set up (FORMAT!) and solve stoichiometry problems (mole ratios, mass-to-mass stoich and limiting/excess reactant problems). 

Answers:

 

SCH 4C - Mole Review

Homework:

 

Answers:

 

SCH 4C - Solution Prep

Preparing a Solution from a Solid

ex. What mass of sodium hydroxide is required to make 500.0 mL of 0.200 mol/L solution?

m = ?                            n = cV                                             n = m/M

V = 0.5000L                    = 0.200 mol/L(0.5000L)               m = 0.100 mol(40.00 g/mol)

c = 0.200 mol/L               = 0.100 mol                                     = 4.00 g

M = 40.00 g/mol

Procedure:

1. Complete calculations to determine mass of solid solute required.
2. Mass out the solid solute on the electronic scale.*
3. Add about 3 cm of distilled H₂O to an appropriately sized volumetric flask.**
4. Place the solid solute into the volumetric flask, using a powder funnel.
5. Dissolve the solid solute by swirling the flask.
6. Fill the flask to the line*** with distilled water and label.****
7. Top up to line when solution reaches room temperature.*****

 Safety:  goggles; gloves and apron when appropriate

Tips and Tricks:

 * If using a hygroscopic solid (a substance that picks up moisture from the air), like sodium hydroxide, measure and place the lid back on the stock bottle asap. 

** A volumetric flask is used because it has a high degree of accuracy.  Other measuring equipment that measures volume accurately includes graduated cylinders, pipettes and burettes.  Equipment that measures volume poorly includes beakers and Erlenmeyer flasks.

*** A volumetric flask has a line etched into the neck too show you where to fill to attain the desired volume.  You must take into consideration meniscus and parallax when filling. 

**** Label should include the your name, as well as the chemical's name, concentration and the date it was prepared. 

***** Many solutions get hot during the dissolution process.  When the solution cools down, the solution level in the flask will drop so you must top it up with distilled water.



Introducing a New Equation
Whether you place 1 mol of a salt into a teacup of water or into a bathtub of water you still have 1 mol of the compound.  Therefore, 

n_concentrated = n_dilute

and since n = cV

c_concentratedV_concentrated = c_diluteV_dilute

or simply written by replacing the letters with numbers,

c₁V₁ = c₂V₂

This equation will be useful in our next calculation.

Preparing a Solution from a Stock Solution

ex. What volume of stock hydrochloric acid (12.0 mol/L) is required to make 1.00 L of 0.150 mol/L solution?

V₁ = ?                                          c₁V₁ = c₂V₂

c₁ = 12.0 mol/L            12.0 mol/L(V₁) = 0.150 mol/L(1.00 L)

V₂ = 1.00 L                                     V₁ = 0.0125 L                         

c₂ = 0.150 mol/L                                 = 12.5 mL

We always convert to mL at the end because all our equipment measures in mL.

Procedure:

1. Complete the calculation to determine the volume of stock solution required.
2. Use a graduated cylinder to measure out the stock solution.
3. Add about 3 cm of dH₂O to the flask.
4. Place the stock solution into a volumetric flask (1.00 L), using a funnel.
5. Dissolve the stock solution.
6. Fill the flask to the line and label.
7. Top up to line when solution reaches room temperature.

 Safety:  goggles, gloves, apron, fume hood

Homework:  

Practice, p. 136 # 20, 21, 22

Section Questions, p. 137 # 1-6

Answers:

SCH 4C - Solutions & Solution Concentration

Solutions

Solutions consist of a solute (the substance that gets dissolved) and a solvent (the substance that does the dissolving).  Solutions are homogeneous.

 

A solution can either be dilute or concentrated, depending on how much solute is dissolved in the solvent – this is a qualitative measure of the concentration of a solution. 

 

We can also get a quantitative measure of concentration:

• percentage concentration – m/V, m/m or m/V
• parts per million (ppm) or parts per billion (ppb)
• molar concentration (usually referred to as simply “concentration’)

Percentage Concentration

Sometimes concentrations are expressed as percentages.  For instance, the label of a vinegar bottle shows 5% V/V acetic acid (meaning there are 5 mL of acetic acid for every 100 mL of vinegar solution).

Percentage concentration is expressed as m/V (mass by volume), m/m (mass by mass) or V/V (volume by volume):

W/V  ↝ c_solution = m_solute/V_solution × 100, where mass is in g and volume is in mL

V/V  ↝ c_solution = V_solute/V_solution × 100, volume is in mL           

ex. A salt solution is formed by mixing 1.50 g of sodium chloride in enough water to make 250 mL of solution.  What is the m/V percentage concentration of salt in the solution?

m_solute = 1.50 g                   c_solution = m_solute/V_solution × 100

V_solution = 250 mL                          = (1.50 g/250 mL) × 100

                                                    = 0.60%

                                         ∴ the salt solution is 0.60% W/V

Parts per Million (Parts per Billion)

This is used when we want to express the concentration of a very dilute solution (ppm – one part solute for every million parts of solution)

ppm ↝ c = m_solute/V_solution × 10⁶, where m is in g and volume is in mL

ppb  ↝ c = m_solute/_Vsolution × 10⁹, where m is in g and volume is in mL

ex. In a chemical analysis, 0.0022 g of oxygen was measured in 250 mL of pond water.  What is the concentration of oxygen in ppm?

m_O2 = 0.0022 g            c_O2 = m_O2/ V_H2O × 10⁶

V_H2O = 250 mL                    = (0.0022 g/250 mL) × 10⁶

c_O2 = ?                                 = 8.8 ppm                     

                                    ∴ the concentration is 8.8 ppm.

Molar Concentration

If we wish to make up a solution of known concentration, we need to know how much solute to measure.

n = cV, where n is the number of moles (mol), c is concentration (mol/L) and V is volume (L)

ex. What amount of lithium bromide is required to make 250 mL of 1.25 mol/L solution?

V = 250 mL                       n = cV

c = 1.25 mol/L                      = (1.25 mol/L)(0.25 L)

n = ?                                   = 0.313 mol                        

                                         ∴ 0.31 mol of lithium bromide is required 

Homework:

Practice, p. 126 # 1, 3

Practice, p. 128 # 5, 6

Practice, p. 130 # 8, 9

Practice, p. 131 # 12, 13

Practice, p. 133 # 16

Answers:

SCH 4U - Alcohols (R-OH) & Ethers (R-O-R)

Alcohols (R-OH) 
All members of the alcohol family will contain the hydroxyl (-OH) functional group.

Some common alcohols are ethanol (in spirits, like wine and beer), cholesterol (a type of fat produced by the bodies of animals) and retinol (vitamin A). 

Naming Alcohols 
The ending ‘-ol’ is added to the name of the parent compound (the longest carbon chain) and a numbering system is used to locate the –OH group when necessary. 

We use the same rules as usual to name alcohols.  1.  Find the longest chain. 2.  Number the main chain to give all substituents (as well as the hydroxyl group) the lowest possible numbers. 3.  List substituents in alphabetical order. 4.  Finish with the main chain, which includes info about the alcohol.  For instance, the third compound is numbered from right to left.  This places methyl groups at carbons 2 and 4 (2,4-dimethyl). The main chain is six carbons long (hex); the carbons in the main chain are singly bonded (an); the hydroxyl group is found at carbon 4 (-4-) and the family is alcohol (ol).

1°, 2° & 3° Alcohols

Alcohols are classified according to the type of carbon atom to which the –OH group is attached.  If this C atom has 1, 2 or 3 alkyl groups attached to it, in addition to the –OH group, the resulting alcohol is said to be primary, secondary or tertiary respectively. In order, the above three compounds are primary, secondary and tertiary.

Polyalcohols

Alcohols that contain more than one –OH group, use prefixes like ‘-diol’ and ‘-triol’ to indicate the number of –OH groups on the molecule.

Sometimes a compound has both a common name (or two) and an IUPAC name.  Notice that when there is more than one OH group, the 'e' at the end of the main chain name comes back into play.  For instance, a two carbon chain with one OH group would be named ethanol.  However, if another OH group is added to the other carbon in this two carbon chain, the name is ethane-1,2-diol (as above).  The reason the 'e' shows back up is because of the old way of naming.  In the old method, antifreeze would have the IUPAC name 1,2-ethanediol (hello, 'e').  It was written like this because it rolls off the tongue better than 1,2-ethandiol (no 'e').  So, even though we use the new system, we maintain some of the nostalgia of the old system.

Cyclic Alcohols

If the –OH group is attached to a ring, it is referred to as a cyclic alcohol.  Many large molecules are known by their common names; menthol and cholesterol to name a pair.  There are also aromatic alcohols, the simplest of which is phenol or hydroxybenzene. 

On the left: cyclopentanol.  It is a cyclopentane with a hydroxyl group attached.  There is no need to locate the OH group because all the carbons in the ring are equivalent.            On the right: phenol or hydroxybenzene.  When benzene is involved in any compound, you are guaranteed to see 'benz' or 'phen' in the name.  So, benzene + OH group = phenol.  It is also IUPAC appropriate to represent the OH group with the name hydroxy (but, only in this case, in high school, we do not use 'hydroxy' at any time other than when attached to benzene).  So, OH group attached to benzene = hydroxybenzene.

Properties of Alcohols 
The properties of the alcohols are based on the nature of the hydroxyl group:

-OH 

• polar and can H-bond
• expect to have higher melting and boiling points for alcohols than their parent alkanes (for instance, look at the major difference in these two bp:  ethane ↝ -89°C; ethanol ↝ 78°C)
• water solubility of alcohols is better than that of the analogous hydrocarbons, due to the H-bonding ability

Reactions Involving Alcohols
(I)  Preparing Alcohols:  Hydration Reactions 
Recall that alkenes readily undergo addition (hydration) reactions.  Starting with an alkene and adding HOH, will introduce the –OH functional group, producing an alcohol.  Just don't forget to apply Markovnikov's rule when necessary.

NameIt!  What are the names of the organic molecules in this reaction?  See the end of the lesson for the answer.

Ethanol can also be prepared by fermentation of sugars, using a yeast culture, in the absence of oxygen: 

C₆H₁₂O₆(s)  →   2CO₂(g)   +   2C₂H₅OH(l)

Methanol, which cannot be produced by an addition reaction, is produced by combining carbon monoxide and hydrogen at a high temperature and pressure in the presence of a catalyst:

CO(g)   +   H₂(g)   →   CH₃OH(l) 

(II)  From Alcohols to Alkenes:  Elimination Reactions 

The addition reaction discussed previously can be carried out in reverse.  Since the elimination reaction results in water removal, it is also called a dehydration reaction. 

The sulfuric acid catalyst causes the removal of water from the alkane.  The OH group and a H on the C adjacent to the hydroxyl-bearing carbon are removed.  The two carbons that lost the OH and H respectively, work together to form a second bond, which results in a double bond.  The OH and H that were removed, join together to form a water molecule as the other product.            NameIt!  What are the names of the organic molecules in this reaction?  See below for the answer.

Ethers (R-O-R′) 
Formerly used as an anaesthetic (diethyl ether).  Prior to the discovery and use of ether in the mid-1800s, surgeries were performed without anaesthesia warning the images at this link are...yikes).  The use of ether revolutionized medicine.

Ethers can either be symmetrical (R-O-R) or asymmetrical (R-O-R′), depending on whether the alkyl groups are the same or different. 

Naming Ethers
Ethers can be named by stating, in alphabetical order, the names of the hydrocarbons followed by ‘ether’ (if the two sides are the same, the prefix ‘di-‘ is used). 

**Note: There are two accepted methods for naming an ether.  I am only going to show you one method - if you wish to know the other, message me.**            Starting at the Cs attached directly to the -O-, number outward in either direction along the longest chains (on the left there is an unsubstituted three C chain and on the right, there is a branched six C chain).  Name both sides as substituents (on the left, we have propyl and on the right we have 3-ethylhexyl).  Place the two in alphabetical order with a space between them: 3-ethylhexyl propyl.  Then add a space and "ether" to give:  3-ethylhexyl propyl ether.            If both substituents are the same alkyl group, the prefix "di" is employed.  For instance, diethyl ether has two ethyl groups attached to the central oxygen.

Properties of Ethers 

The properties of the ethers are based on the characteristics imparted by the -O- within the hydrocarbon chain:

-O-

•  C-O-C bonds are polar, but cannot H-bond
• expect to have higher mp, bp and water solubility than analogous hydrocarbons (due to polarity)
• expect to have lower mp, bp and water solubility than analogous alcohols (due to loss of H-bond ability)

 

 

Preparing Ethers from Alcohols:  Condensation Reactions 

Ethers are formed by the reaction of two alcohol molecules, which eliminates a water molecule. 

The sulfuric acid catalyst removes H from the hydroxyl group of one alcohol (leaving CH₃CH₂O-) and the OH group from the other alcohol (leaving -CH₂CH₂CH₂CH₃).  This allows the two pieces to link together CH₃CH₂O─CH₂CH₂CH₂CH₃ and form an ether.  The eliminated H and OH combine to form the other product, HOH.            NameIt! What are the names of the organic molecules in this reaction?  See the end of the lesson for the answer.

NameIts! Answers: 
cis-but-2-ene, butan-2-ol
propan-2-ol, propene
ethanol, butan-1-ol, butyl ethyl ether

Homework #21-35